PROJECT SUMMARY Melanoma is one of the most prevalent cancers and is estimated to account for over 10,000 deaths in 2016 in the United States alone. Although highly curable when detected early, the 5-year survival rates decline precipitously for patients with regional (62.4%) and metastatic (17.9%) melanoma indicating a lack of efficacy of conventional therapies, and a strong need for novel therapies for this patient population. In response to this need, we have engineered a nanoimmunotherapy for treating melanoma, which combines Prussian blue nanoparticle-based photothermal therapy (PBNP-PTT) with immune checkpoint inhibition. Immune checkpoint inhibitors represent a class of monoclonal antibodies that target key immune checkpoints, which have resulted in dramatic treatment responses in previously untreatable cancers, particularly metastatic melanoma. Despite this immense promise, responses to immune checkpoint inhibitors are limited to small subsets of patients and are associated with immune-related adverse events, some of which can be lethal. Thus, there is great need to increase the efficacy of immune checkpoint inhibitors so that they elicit responses in a greater proportion of treated patients. Our approach toward achieving this end is to engineer a nanoimmunotherapy which combines PBNP-PTT with anti-PD-1 and anti-CTLA-4. In our nanoimmunotherapy (PBNP-PTT plus anti-PD-1 plus anti- CTLA-4), PBNP-PTT serves the dual purpose of ablating the primary tumor to decrease tumor burden and releasing tumor antigens to provide a ?multi-antigen vaccination effect.? These effects are complemented by the administration of immune checkpoint inhibitors, which decrease tumor immunosuppression and elicit a potent antitumor immune response. We anticipate that the interplay between the local immunomodulatory effects of PBNP-PTT with the reversing of immunosuppression elicited by the antibodies will result in complete response and long-term survival in a larger proportion of patients with melanoma. The goal of this project is to test the feasibility of using this nanoimmunotherapy for treating melanoma in a syngeneic mouse model of the disease. In Aim 1, we seek to demonstrate the efficacy of the nanoimmunotherapy in securing effective tumor eradication and preventing relapse. In Aim 2, we seek to elucidate the immunological responses elicited by the nanoimmunotherapy. Successful completion of this project will facilitate further preclinical development of our nanoimmunotherapy in animal models of metastatic melanoma, and ultimately clinical testing and commercialization of our treatment regimen.